U.S. patent number 6,637,871 [Application Number 10/030,671] was granted by the patent office on 2003-10-28 for droplet generator for a continuous stream ink jet print head.
This patent grant is currently assigned to VideoJet Technologies, Inc.. Invention is credited to Andrew D King, Graham D Martin, Sukbir S Pannu, Nigel E Sherman.
United States Patent |
6,637,871 |
Martin , et al. |
October 28, 2003 |
Droplet generator for a continuous stream ink jet print head
Abstract
A droplet generator for a continuous stream ink jet print head
includes an elongate cavity for containing the ink, nozzle orifices
in a wall of the cavity for passing ink from the cavity to form
jets. The nozzle orifices extend along the length of the cavity. An
actuator is disposed on the opposite side of the cavity to the wall
for vibrating the ink in the cavity by itself vibrating relative to
the wall. The vibration is such that each jet breaks up into ink
droplets at the same predetermined distance from the wall of the
cavity. The thickness of the wall through which the nozzle orifices
extend is less than 90 .mu.m. The wall includes a planar member
secured to the remainder of the droplet generator so as to form a
boundary which extends around the nozzle orifices and within which
the planar member is unsupported. The boundary includes first and
second boundary lengths which extend along the length of the cavity
on either side of the nozzle orifices. The distance between the
first and second boundary lengths being less than 1700 .mu.m.
Inventors: |
Martin; Graham D (Sawston,
GB), Sherman; Nigel E (Woolpit, GB), Pannu;
Sukbir S (Grantchester, GB), King; Andrew D (St.
Ives, GB) |
Assignee: |
VideoJet Technologies, Inc.
(Wood Dale, IL)
|
Family
ID: |
10857242 |
Appl.
No.: |
10/030,671 |
Filed: |
May 16, 2002 |
PCT
Filed: |
July 07, 2000 |
PCT No.: |
PCT/GB00/02619 |
PCT
Pub. No.: |
WO01/03933 |
PCT
Pub. Date: |
January 18, 2001 |
Foreign Application Priority Data
|
|
|
|
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Jul 14, 1999 [GB] |
|
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9916532 |
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Current U.S.
Class: |
347/73;
347/75 |
Current CPC
Class: |
B41J
2/025 (20130101); B41J 2/162 (20130101); B41J
2/1632 (20130101) |
Current International
Class: |
B41J
2/025 (20060101); B41J 2/015 (20060101); B41J
2/135 (20060101); B41J 2/16 (20060101); B41J
002/025 (); B41J 002/02 () |
Field of
Search: |
;347/47,48,73,77,74,75 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 389 738 |
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Oct 1990 |
|
EP |
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05330062 |
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Dec 1993 |
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JP |
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07148930 |
|
Jun 1995 |
|
JP |
|
WO 95/25637 |
|
Sep 1995 |
|
WO |
|
WO 96/31289 |
|
Oct 1996 |
|
WO |
|
WO 98/51503 |
|
Nov 1998 |
|
WO |
|
Primary Examiner: Vo; Anh T. N.
Attorney, Agent or Firm: Kirschstein et al.
Claims
What is claimed is:
1. A droplet generator for a continuous stream ink jet print head,
comprising: a) an elongate cavity for containing the ink; b) nozzle
orifices in a wall of said cavity for passing ink from the cavity
to form jets, said nozzle orifices extending along the length of
the cavity; and c) actuator means disposed on the opposite side of
said cavity to said wall for vibrating the ink in the cavity by
itself vibrating relative to said wall to break each said jet up
into ink droplets at a same predetermined distance from said wall
of the cavity, said wall through which the nozzle orifices extend
having a thickness less than 90 .mu.m, said wall comprising a
planar member secured to a remainder of said droplet generator so
as to form a boundary which extends around the nozzle orifices and
within which said planar member is unsupported, said boundary
including first and second boundary lengths which extend along the
length of said cavity on either side of the nozzle orifices, the
first and second boundary lengths being spaced by a distance less
than 1700 .mu.m.
2. The generator according to claim 1, wherein said distance
between said first and second boundary lengths is less than 1350
.mu.m.
3. The generator according to claim 1, wherein said planar member
is a planar metallic member.
4. The generator according to claim 3, wherein said planar metallic
member is stainless steel foil.
5. The generator according to claim 3, wherein said planar metallic
member is secured to the remainder of said droplet generator by
means of welding which extends along a path defining said boundary
around the nozzle orifices.
6. The generator according to claim 5, wherein the nozzle orifices
are electro-discharged machined in the planar metallic member.
7. The generator according to claim 1, wherein said thickness of
said wall through which said nozzle orifices extend is greater than
45 .mu.m.
8. The generator according to claim 1, wherein said thickness of
said wall through which said nozzle orifices extend is greater than
55 .mu.m.
9. The generator according to claim 1, wherein said thickness of
said wall through which said nozzle orifices extend is from 60 to
80 .mu.m.
Description
BACKGROUND OF THE INVENTION
This invention relates to a droplet generator for a continuous
stream ink jet print head.
More particularly the invention relates to such a generator
comprising: an elongate cavity for containing the ink; nozzle
orifices in a wall of the cavity for passing ink from the cavity to
form jets, the nozzle orifices extending along the length of the
cavity; and actuator means disposed on the opposite side of said
cavity to said wall for vibrating the ink in the cavity by itself
vibrating relative to the wall, the vibration being such that each
jet breaks up into ink droplets at the same predetermined distance
from the wall of the cavity. Droplet generators of the aforegoing
type will hereinafter be referred to as droplet generators of the
specified type.
In order to enable generators of the specified type to be used with
corrosive (non-aqueous based) ink, certain known such generators
are constructed predominantly of stainless steel components. One
such component is the wall containing the nozzle orifices, and
takes the form of a thin sheet of stainless steel foil through
which the orifices extend.
The orifices have to be comparatively small and of very high
quality. This is so that the jets produced by the orifices are
identical. They must be parallel to one another to fractions of a
degree, and have equivalent velocities to within a few percent.
This requires perfectly round holes with relative sizes to within 5
percent. There are few fabrication techniques that can achieve this
requirement in stainless steel. All techniques suffer and encounter
increasing difficulty as the thickness of the foil increases. The
superior technique evolved is electro discharge machining
(EDM).
In such an orifice formation process a thin metal wire or electrode
is brought to within close proximity of the foil. A voltage is
applied across the gap and as arcing occurs between the foil
workpiece and the electrode local heating results in vaporisation
and expulsion of the foil material. In order to achieve holes of
the required quality very low current is applied. This improves the
finish of the holes but increases the time required to `drill` each
hole, and hence complete the drilling of the full array of
holes/orifices. In a 128 dots per inch (DPI) printer having a 50 mm
long line of 256 holes, each extending through foil 100 .mu.m
thick, the drilling time amounts to 12-13 hours. This time is
considerable and has significant production implications with
respect to both unit cost and capacity.
The measure of an ink jet printer's ability to print on distant
substrates is termed the `throw` of the printer. A high throw is
necessary when printing on uneven substrates or in conditions where
there is significant air turbulence in the region of the jets.
Throw is related to jet velocity. Jet velocity equals wavelength
multiplied by frequency. Vibration of the actuator means at the
frequency of operation of the generator produces an ultrasonic wave
which travels down the jets. This wave is clearly visible in the
jets under suitable magnification, and enables wavelength and
therefore jet velocity to be measured. For a given frequency of
operation, wavelength can be used as a measure of jet velocity and
hence throw of a printer. It can be seen that at a given frequency
of operation it is desirable to maximise jet wavelength to maximise
throw.
In a known ink jet printer, having a standard 128 DPI nozzle
produced in 100 .mu.m thick stainless steel foil, when using
methylethylketone ink, the operating range of wavelengths is 155 to
165 .mu.m, giving a mean operating wavelength of 160 .mu.m
representing a jet velocity of 12 m/s.
SUMMARY OF THE INVENTION
According to the present invention there is provided a droplet
generator for a continuous stream ink jet print head comprising: an
elongate cavity for containing the ink; nozzle orifices in a wall
of said cavity for passing ink from the cavity to form jets, said
nozzle orifices extending along the length of said cavity; and
actuator means disposed on the opposite side of said cavity to said
wall for vibrating the ink in said cavity by itself vibrating
relative to said wall, the vibration being such that each said jet
breaks up into ink droplets at the same predetermined distance from
said wall of the cavity, the thickness of said wall through which
said nozzle orifices extend being less than 90 .mu.m, said wall
comprising a planar member secured to the remainder of said droplet
generator so as to form a boundary which extends around said nozzle
orifices and within which said planar member is unsupported, said
boundary including first and second boundary lengths which extend
along the length of said cavity on either side of the nozzle
orifices, the distance between said first and second boundary
lengths being less than 1700 .mu.m.
Preferably, the distance between said first and second boundary
lengths is less than 1350 .mu.m.
Preferably, said planar member is a planar metallic member, e.g.
stainless steel foil. Preferably, said planar metallic member is
secured to the remainder of said droplet generator by means of
welding, the path taken by the welding defining said boundary
around the nozzle orifices. Preferably, the nozzle orifices have
been formed in said planar metallic member by electro discharge
machining.
Preferably, said thickness of said wall through which said nozzle
orifices extend is greater than 45 .mu.m, more preferably greater
than 55 .mu.m, even more preferably from 60 to 80 .mu.m.
BRIEF DESCRIPTION OF THE DRAWINGS
A droplet generator in accordance with the present invention will
now be described, by way of example, with reference to the
accompanying drawings, in which:
FIG. 1 is a front view of the generator;
FIG. 2 is a side view of the generator of FIG. 1;
FIG. 3 is an underneath view of the generator of FIG. 1;
FIG. 4 is a graph of resonant frequency vs. thickness of a nozzle
orifice foil sheet of the generator of FIG. 1;
FIG. 5 is a graph of resonant frequency vs. free unsecured width of
the foil sheet of the generator of FIG. 1;
FIG. 6 is a graph of thickness vs. free unsecured width of the foil
sheet of the generator of FIG. 1, showing the combinations of
thickness and unsecured width which give rise to resonance of the
foil sheet at four different frequencies; and
FIG. 7 is a graph of ink jet misdirection vs. foil sheet thickness
of the generator of FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIGS. 1 to 3, the generator comprises a stainless
steel manifold 1, a stainless steel spacer 2, an actuator 3 and a
stainless steel nozzle carrier 5. Actuator 3 comprises a
piezoelectric driver 9, a stainless steel head 11 and a brass
backing member 6, and is held within manifold 1 by means of a
compliant element 8. Piezoelectric driver 9 is driven by means of a
single electrical connection to brass backing member 6 and the
earthing of steel head 11. Nozzle carrier 5 comprises a stainless
steel element 4 defining therein a `V` cross section channel, and
secured to element 4, a stainless steel foil sheet 10. Sheet 10
contains a line of nozzle orifices 7, and is so secured to element
4 that this line runs along the length of the open apex of the `V`
cross section channel of element 4. Manifold 1, spacer 2 and nozzle
carrier 5 are bolted together. Foil sheet 10 is welded to nozzle
carrier 5. FIG. 3 shows the path 12 of the weld. Since practically
all adhesive based bonding techniques are incompatible with the use
of corrosive ink, the absence of such bonding techniques in the
generator enables the use, if desired, of such ink. It is to be
noted that due to the thickness of foil sheet 10 (see later), it is
not possible to diffusion bond or braze sheet 10 to carrier 5,
since such techniques would cause unacceptable distortion of sheet
10.
An elongate ink cavity 13 is defined by the lower face 15 of
actuator 3 and interior faces 17, 19 of element 4 and spacer 2. A
narrow gap 20 is present on either side of head 11 of actuator 3
between it and manifold 1. `O` rings (not shown) just below
compliant element 8 seal against the further eggression of ink from
cavity 13 and gaps 20. Thus, piezoelectric driver 9 is sealed from
contact with the ink. Channels (not shown) are provided in manifold
1 and communicate with gaps 20 for the supply of ink to cavity 13
and the bleeding of air/ink from cavity 13.
At the frequency of operation of the generator, cavity 13 has a
resonant frequency at which ink within cavity 13 immediately
adjacent the line of nozzle orifices 7 vibrates in phase and with
the same amplitude in a direction perpendicular to the plane of
foil sheet 10 containing nozzle orifices 7. Thus, the vibration of
the ink in cavity 13 is such that each ink jet breaks up into ink
droplets at the same predetermined distance from its respective
nozzle orifice 7.
It is a requirement for proper operation of the generator that
there be comparatively low communication of the vibration of
actuator 3 to other generator structure on the boundary of ink
cavity 13. Indeed, the design intent is that actuator 3 execute a
piston like motion within the surrounding stationary structure of
the generator. The foregoing leads to the requirement that the
frequency of excitation applied to actuator 3 must be sufficiently
distant from resonant frequencies of foil sheet 10. The droplet
generator is designed to operate at a frequency sufficiently below
the first resonance mode of foil sheet 10.
The frequency of this first resonance mode is related to the
thickness of sheet 10. Reference is to be made here to the graph of
FIG. 4. The thicker foil sheet 10 the higher its first resonant
frequency. Droplet generators capable of operating at high
frequencies of excitation allow fast print speeds, an important and
desirable characteristic. Thus, in a given droplet generator there
is a limit on the minimum thickness of foil sheet 10 for a given
operating frequency.
It is possible to overcome this limit on the thickness of foil
sheet 10 by modifying the geometry of the attachment of sheet 10 to
nozzle carrier 5 as will now be explained.
Foil sheet 10 is secured to nozzle carrier 5 along weld path 12.
The region of sheet 10 inside weld path 12 is unsupported except
for its boundaries with the weld. This forms a long thin sliver of
unsupported foil. The width of this sliver is defined as the foil
free-width a (see FIG. 3). Mathematical analysis of foil resonance
shows that the frequency of the foil's first resonance mode is
related not only to the thickness k of the foil, but also to the
width a and length b of the sliver of unsupported foil. Resonant
frequency F=(c/2).(sqrt((n.sup.2 /a.sup.2)+(m.sup.2 /b.sup.2))),
where n and m are mode numbers, and c is wave speed and is given by
c=w.sup.1/2, where w is the pulsatance, E is Young's modulus, v is
Poisson's ratio, and p is density. Thus, it will be seen that the
narrower the foil free width a the higher the resonant frequency.
Reference is to be made here to the graph of FIG. 5 (drawn for a
foil 45 .mu.m thick).
The aforegoing analysis reveals that it is possible to work with
foils thinner than previously thought possible, by modifying the
geometry of the attachment of the foil to the nozzle carrier,
specifically by modifying the foil free width a. Reference is to be
made here to the graph of FIG. 6. In the graph four curves are
plotted each representing an operating frequency (50, 75, 100, 125
kHz) of the generator, and hence each representing a foil resonance
frequency to be avoided by the choice of an appropriate foil
thickness and width according to the graph. In the graph, for each
operating frequency, the region of foil thickness/width
combinations well above and well to the left of the line
representing the frequency are acceptable thickness/width
combinations. It is to be noted that as the frequency of operation
decreases, the size of the region of acceptable thickness/width
combinations increases. Thus, it will be seen that, dependent on
the frequency of operation, there is an infinite number of foil
thickness/width combinations that can be chosen to avoid foil first
mode resonance problems.
In continuous array ink jet printing one design aim is that the
jets be `satellite` free, i.e. that the `proper` droplets of each
droplet stream are not interposed with much smaller so called
satellite droplets. Also, as already stated, it is required that
each jet break up into droplets at the same predetermined distance
from its respective nozzle orifice. It has been found that the
thinner the foil sheet 10 the higher the wavelength required to
best meet these two criteria. Since, as explained previously, for a
given frequency of operation, wavelength can be used as a measure
of printer throw, it can be seen that the consequence of reducing
the thickness of foil sheet 10, is to increase printer throw.
The thinner foil sheet 10 the less time taken to drill the line of
nozzle orifices 7 using EDM. EDM is a high quality but
comparatively slow machining process. Due to material clearance
requirements, the EDM process becomes slower as hole depth
increases. In general drilling time is related to the square of the
drilling depth, i.e. if drilling depth is increased by a factor of
sqrt 2, drilling time is doubled. It will be apparent that even a
small reduction in the thickness of foil sheet 10 confers a
significant gain in terms of orifice drilling time.
Jet misdirection is an expression used to describe the case where
ink jets emanate from nozzle orifices 7 in directions other than
intended. Jet misdirection is related to the thickness of foil
sheet 10. Thicker foils tend to offer better jet directionality
since any lack of uniformity in flow entering an orifice tends to
be corrected by the orifice itself as the flow travels along its
length. The boundary layer of flow immediately adjacent the orifice
wall grows in thickness downstream of entry into the orifice and
eventually forms a fully developed flow, somewhat independent of
input conditions. Jet directionality is key to high quality prints.
Any small misalignments between jets causes imperfections in print
samples that can be unacceptable.
Finite element analysis modelling work suggested that the
relationship between good jet directionality and foil thickness was
a non-linear one. It appeared to asymptote rapidly towards skewed
jet arrays at low foil thickness. The susceptibility of a jet to a
given flow irregularity was investigated and showed that, for the
flow conditions in a typical continuous array ink jet print head,
the jet direction error asymptotes to zero near 100 .mu.m in foil
thickness. The degradation in jet array quality due to any
reduction in foil thickness would therefore be gradual near 100
.mu.m and increase rapidly as the foil thickness approaches zero.
Reference is to be made here to the graph of FIG. 7. There appeared
to be a breakpoint at around 45 .mu.m foil thickness. This suggests
that by working above 45 .mu.m minimal reduction in print quality
due to jet misalignment effects would be experienced.
Comment will now be made regarding issues associated with welding
of foil sheet 10 to nozzle carrier element 4.
Welding as a process has distortion issues associated with thin
foils. The heat generated by the welding process must not be
allowed to deform the bulk of the foil as these deformations will
affect subsequent jetting. Further, the welding process requires
good contact between the foil and the nozzle carrier, and
distortion compromises this. In general the welding of thinner
foils is limited due to its greater susceptibility to these heating
effects.
The foil is welded accross a thin (300 .mu.m) slot in the stainless
steel nozzle carrier. The slot is defined by the aforementioned
open apex of the `V` cross section channel of element 4 of nozzle
carrier 5, and is labelled 25 in FIG. 3. The slot is made as narrow
as possible but must be wide enough to offer little disturbance to
ink entering the nozzle holes. The turbulence associated with the
flow along the edge of the slot and the slot/foil interface can
cause jet directionality problems. The foil welding process is
critical to this. It requires good contact between the foil and the
nozzle carrier and uniform heat dispersion from the foil into the
carrier. This tends to restrict the minimum distance permissible
between the weld path and the edge of the slot. These difficulties
restrict the position of the weld beads holding the foil to the
carrier and limit the minimum free unsecured width of the foil. The
welding process has the tendency to produce dross and debris. The
region of the foil near the holes has to be kept clear of this
debris or again directionality problems can occur. The closer the
weld to the nozzle holes, the greater the risk of dross associated
problems.
It will be seen from the foregoing that welding associated issues
place a limitation on the minimum thickness of the foil and the
minimum foil free width. Obviously, the quality of the welding
process used in a given case is relevant to the determination of
the particular limits on foil thickness and foil free width in that
given case.
In the light of the above analyses/understandings, a range of
thicknesses of foil sheets 10 were tried in the droplet generator
of FIGS. 1 to 3. The range tried was 45, 55, 65, 75, 85, 95 and 100
.mu.m, and in the case of each thickness the foil free width used
was 500 .mu.m. The foils were drilled with standard 128 DPI holes.
Drilling times for thinner foils were significantly quicker. In
particular, 65 .mu.m foil drilling times were 5-6 hours compared
with 12-13 hours for 100 .mu.m foil. The thinner foil nozzles were
jetted under a variety of conditions. These included a range of
wavelengths, print heights and print speeds. It was found that
although the jet straightness and subsequent drop positioning did
suffer with reduced foil thickness the effects were only really
apparent in 45 .mu.m foils. Due to foil resonance problems, foils
at 55 .mu.m thickness and thinner failed to produce uniform jet
break-off across the jet array, in conditions acceptable to thicker
foils. Nozzles which satisfied this criteria, 65 .mu.m and thicker,
were run at a range of wavelengths with solvent based ink. In
particular, the arrays were assessed by their ability to satisfy
the satellite free condition and uniform break-up length.
Conditions were chosen which maximised the satellite free condition
and uniform break-up length for each foil thickness.
65 .mu.m foil was found to give optimum results at wavelength 170
to 180 .mu.m giving an operating mean of 175 .mu.m. This compares
to a mean operating wavelength of 160 .mu.m for 100 .mu.m foil.
This represents a change in jet velocity from 12 m/s to 13.125 m/s.
This is a desirable 9% increase in jet velocity with a
corresponding improvement in throw. It is believed that the
increase in jet velocity with thinner foil is due to improved fluid
flow characteristics, e.g. the development of the dynamic flow
profile within each orifice.
In the aforedescribed, lower limits on foil thickness of 45 .mu.m
and 55 .mu.m are mentioned. The 45 .mu.m limit is due to jet
misalignment problems. The 55 .mu.m limit is due to foil resonance
problems. It is to be appreciated that it is possible to lower
these limits by making refinements in the droplet generator/print
head, e.g. better quality welding of foil to nozzle carrier (see
earlier), narrowing of foil free width, improvement in electro
discharge machining of nozzle orifices to provide better orifice
geometry, improving uniformity in flow entering nozzle orifices,
and lowering in operating frequency (see FIG. 6).
The droplet generator described above by way of example is one of
the specified type designed so that its ink cavity is resonant at
operating frequency. It is to be understood that the present
invention is also applicable to a droplet generator of the
specified type designed so that its actuator is resonant at
operating frequency.
* * * * *